1. Field of the Invention
The present invention relates to a method for manufacturing a colorless transparent polyimide film for flexible displays, and more particularly, to a method for increasing the optical transmittance of a polyimide substrate with glass fabric reinforced for flexible displays.
2. Description of the Related Art
Various flat panel displays using such techniques as e-paper displays (EPD), plasma displays (PDP), liquid crystal displays (LCD), and organic light emitting displays (OLED) are being utilized in TV sets, mobile phones, monitors, e-books, and mobile devices, among others.
In next generation, however, flexible displays which are portable and convenient to use regardless of time and place, are expected to be widely employed in electronic devices, such as mobile phones, portable terminals, and laptop computers.
A substrate for flexible displays has to possess mechanical flexibility for flexible displays to be easily bendable, rollable, or foldable. While a flat panel display uses a glass substrate, flexible displays should have a mechanically flexible substrate. A very thin glass plate, a thin stainless steel plate, or a plastic film may be used as a flexible substrate, but a very thin glass plate and a stainless steel plate have limitations in terms of flexibility. Thus, the use of a plastic film is the most favorable.
A plastic film substrate has significantly higher mechanical flexibility but has inferior tensile strength compared with glass substrates. When glass fabric is reinforced in a plastic film, the tensile strength of the plastic film may be enhanced as fiber-reinforced plastic.
With the goal of manufacturing a display device, a thin film transistor (TFT) should be provided on a substrate to control the switching and luminance of individual pixels. Currently available TFTs use amorphous silicon, oxide, and organic materials, among others. In the case of amorphous silicon, which exhibits very stable performance, the minimum processing temperature necessary for deposition and thermal treatment is approximately 230° C. When a plastic substrate is subjected to a TFT thin film process at 230° C. and then cooled to room temperature, the TFT thin film may be stripped from the plastic substrate because of a difference in the coefficient of thermal expansion (CTE) between the plastic substrate and the TFT thin film material. To prevent this occurrence, the CTE of the substrate should be approximately 10 ppm/° C.
The CTE of plastic is typically at least 50 ppm/° C. When glass fabric with a CTE of approximately 5 ppm/° C. is reinforced in plastic film, the CTE of the substrate may be reduced to about 10 ppm/° C.
For this reason, glass fabric is reinforced in plastic film to enhance the thermal and mechanical properties of a substrate for a flexible display. In this case, however, if the refractive index of the glass fabric differs from that of the plastic film, light may be scattered by the glass fabric, such that the substrate becomes optically opaque. This condition prevents the use of the above substrate for a flexible display.
Opacity increases in proportion to an increase in the difference in refractive index between the glass fabric and the plastic film. The substrate for a flexible display has to have an optical transmittance of approximately 85% or more. Thus, the refractive indexes of the glass fabric and the plastic film should be maximally matched to ensure the high optical transmittance and transparency of the substrate.
The refractive index of a typical plastic resulting from polymerization of monomers is determined by the monomer. Unless the refractive index of a plastic made of a given monomer coincides with that of the glass fabric, matching the refractive index of a given plastic to that of glass fabric becomes very difficult.
To solve such problems, techniques for adjusting the refractive index of a plastic by dispersing metal nanoparticles in the plastic have been proposed (S. Kubo, A. Diaz, Y. Tang, T. S. Mayer, I. C. Khoo, T. E. Mallouk, Nano Letters, vol. 7, 3418-3423, 2007; S. Mahendia, A. K. Tomar, P. K. Goyal, S. Kumar, J. Appl. Phys. vol. 113, 073103, 2013; S. K. Medda, M. Mitra, S. De, S. Pal, G. De, PRAMANA-Journal of Physics, vol. 65, 931-936, 2005; Korean Patent Application Publication No. 10-2011-0000195 “Plastic substrate and Device including the same”, etc.). However, the problem of remarkably reduced optical transmittance attributed to light absorption by metal nanoparticles occurs. In this case, the use of such plastic as a substrate for flexible displays becomes difficult.